In a mobile communication system including a plurality of cells, a mobile station UE (User Equipment) is configured to continue a communication by switching the cells, when moving from one cell to another cell. Such a cell switch is called “Mobility control”, more specifically, called “handover”.
Generally, in the mobile communication system, when a mobile station UE moves to a peripheral cell, and when a received power of a signal from the peripheral cell becomes stronger than a received power of a signal from a serving cell in the mobile station UE, the mobile station UE is configured to perform the handover to the peripheral cell.
That is, the mobile station UE needs to measure the received power of the signal from the serving cell and the peripheral cell simultaneously of transmitting to and receiving data to and from the serving cell.
It is noted that the received power of the signal from the peripheral cell or the serving cell is “received power of a reference signal of a downlink (RSRP: Reference Signal Received Power)” transmitted from the peripheral cell or the serving cell, for example.
With reference to FIG. 1, one example of the handover procedure will be specifically explained.
As illustrated in FIG. 1, in step S1, the mobile station UE measures the received power of the signal from the serving cell and the peripheral cell.
In step S2, the mobile station UE determines whether or not the received power of the signal from the peripheral cell satisfies the following (Equation 1).(received power of signal from peripheral cell)+(hysteresis)>(received power of signal from serving cell)   (Equation 1)
When it is determined that the (Equation 1) is satisfied, the mobile station UE notifies, to a network, an event A3 for reporting the above-described measurement result, in step S2.
In this case, as a received power (radio quality) Fn of a signal, values calculated by the following (Equation 2) and (Equation 3) are to be used.
Specifically, in the mobile station UE, an upper layer is configured to perform a filtering process (L3 Filtering) represented by (Equation 2), on a measurement value by a physical layer.Fn=(1−a)·Fn-1+a·Mn   (Equation 2)a=½(k/4)   (Equation 3)
It is noted that a value “k” in (Equation 3) is to be notified in advance from the radio base station to the mobile station UE.
Further, in (Equation 2), “n” denotes an index relating to a measurement timing, “Fn” denotes a measurement result after filtering, “Fn-1” denotes a measurement result after filtering at a measurement timing one before, and “Mn” denotes a measurement result in the measurement unit.
In step S3, when the network receives a notification of the event A3, the mobile station UE determines to perform a handover on the cell relating to the received event A3.
In a mobile communication system of the LTE (Long Term Evolution) scheme, the DRX (Discontinuous Reception) control is applied.
The discontinuous reception control is applied when the radio base station eNB and the mobile station UE are in communication and there is no data that should be communicated, and the mobile station UE in a discontinuous reception state is configured to receive periodically, i.e., discontinuously, a downlink control signal transmitted via a PDCCH (Physical Downlink Control Channel).
A time at which the downlink control signal transmitted via the PDCCH is received is called “On-duration (reception interval)”.
In such a case, since the mobile station UE has only to receive the downlink control signal transmitted via the physical downlink control channel PDCCH discontinuously, instead of at all timings, the power consumption of the battery can be reduced.
More specifically, as illustrated in 2, the mobile station UE is configured to receive the downlink control signal transmitted via the physical downlink control channel PDCCH only at the reception interval (5 ms in the example of FIG. 2) set in each DRX cycle (1280 ms in the example of FIG. 2) and other transceivers are configured to be turned off. As a result, the power consumption of the battery of the mobile station UE can be reduced.
It is noted that in the discontinuous reception state, in order to maximize the effect that the power consumption of the battery is reduced, the frequency of measuring the received power of the signal from the serving cell and the peripheral cell is reduced.
As described above, the mobile communication system of the LTE scheme adopts the discontinuous reception control when the radio base station eNB and the mobile station UE are in connection. That is, the state of each mobile station UE includes two types of states a discontinuous reception state and a non-discontinuous reception state according to the presence or absence of data to be communicated.
Here, the mobile station UE in the discontinuous reception state is generally configured to measure the serving cell and the peripheral cell only in the reception interval in the discontinuous reception control, in order to maintain the battery saving effect by the discontinuous reception control.
Generally, the measurement interval of the received power of the signal from the serving cell and the peripheral cell in the discontinuous reception state (hereinafter, referred to as “measurement interval in the discontinuous reception state”) is longer than a measurement interval of the received power of the signal from the serving cell and the peripheral cell in a non-discontinuous reception state (hereinafter, referred to as “measurement interval in a non-discontinuous reception state”).
This is due to the fact that in the discontinuous reception state, the mobile station UE measures the received power of the signal from the serving cell and the peripheral cell only in the reception interval, as described above, and thus, there is a problem that a measurement sample number decreases, as a result of which in order to solve this problem, the measurement sample number is increased to improve the measurement accuracy.
For example, the measurement interval in a non-discontinuous reception state is 200 ms, and the measurement interval in the discontinuous reception state takes a value obtained by making the DRX cycle five times greater. In this case, making the DRX cycle five times greater means equally to a value obtained by averaging five measurement results in the reception interval being resulting in the measurement result in the discontinuous reception state.
However, resulting from the fact that the measurement interval differs between the non-discontinuous reception state and the discontinuous reception state, the following problem occurs in the above-mentioned filtering process.
For example, now consider a case where an optimal value of “k” in the non-discontinuous reception state is “4”. In this case, the above-described (Equation 2) is changed to:Fn=0.5·Fn-1+0.5·Mn   (Equation 4)
As a result, based on the fact that the contribution of “Mn” that is the measurement result before filtering is half, an approximate measurement interval after filtering may be 400 ms. This value, 400 ms, is calculated by 200 ms÷0.5.
On the other hand, in the discontinuous reception state, when the DRX cycle is assumed to be 1280 ms and the above-described (Equation 4) is directly applied, an approximate measurement interval after filtering may be 12800 ms. This value, 12800 ms, is calculated by 6400 ms (1280 ms×5)÷0.5.
Generally, in the handover or other Mobility controls, when the handover or the other Mobility controls are delayed, it is not possible to communicate with the optimal cell, and thus, the communication quality deteriorates.
Therefore, in the above-mentioned discontinuous reception state, there is no problem that in order to improve the accuracy of the measurement result, the measurement is carried out in the averaging interval where the DRX cycle is made five time greater; however, the problem lies in the process of further filtering the measurement result measured in the averaging interval where the DRX cycle is made five time greater triggers the delay of the handover or other Mobility controls.
Therefore, the present invention is intended to overcome the above-described problem. An object of the present invention is to provide a mobile station enabling appropriate measurement of a peripheral cell and an appropriate handover when discontinuous reception control is applied, and a mobile communication method therefore.